This invention relates to an apparatus for recharging a battery, comprising a compartment for accommodating the battery in a manner such that its electrical terminals are in contact with a pair of electrodes, the electrodes being connectable to corresponding poles of a controllable source of electrical energy, further comprising a strain gauge which is positioned so as to make contact with a wall of the battery when the battery is in place in the compartment, the strain gauge comprising a foil which carries a resistive element, each of the two extremities of the resistive element being connected to electrical means via a separate contact wire.
The invention also relates to a method of monitoring both the mechanical deformation and the temperature of a battery during recharging.
The term "battery" is here intended to refer to either single cells or battery packs, and refers specifically to secondary (i.e. rechargeable) batteries.
An apparatus as specified in the opening paragraph is known from U.S. Pat. No. 5,260,638 and from laid-open Japanese Patent Applications JP 63-268445 (filed 24.04.87) and JP 5-326027 (filed 22.05.92). These documents describe how the physical dimensions of a battery can change as a result of increased internal pressure and temperature during charging. Such increase is attributable to the occurrence of certain chemical reactions within the battery, the rate and type of which are determined by the battery's charge state at any given time. In particular, at the end of a charging procedure, additional electrical energy supplied to the battery will, in general, no longer cause an increase in its internal charge, but will instead provoke a sharp rise in the battery's internal pressure and temperature, causing the battery to expand (these effects being particularly prominent in NiCd and NiMH batteries). The strain gauge is employed to monitor any such mechanical deformation of the battery, and the detection of (the onset of) sharply increasing deformation can then be used as an indication that efficient charging has been completed, and that the electrical energy source should be switched off. Such action is particularly important in the case of so-called "quick charging", where the battery can be severely damaged if the charging process is not carefully monitored and interrupted in good time.